Pandorum/WMG

Gallo convinced the passengers to become cannibals

Gallo tried this at the end of the film with Bower. He would wake up the passengers and tell them what happened to Earth so they can suffer from Pandorum in order to preach his views on life to them. We've seen a guy with Pandorum becoming convinced that a ship was cursed, it would be pretty easy to convince someone with Pandorum that humanity's ways of thinking were the cause of Earth's overpopulation and that living in a natural animalistic/primitive state was the way to go. That's what Leland meant by when he said "he cast out those who behave" referring to the passengers with Pandorum.

The albino mutants are distant descendants of crew awakened centuries ago

Mutations that drastic can't have happened to an adult organism, jumpstarted evolution or not. When the Elysium landed it automatically woke a couple hundred colonists and they couldn't escape the ship so they stayed and evolved into albino cannibals.

- Actually, I believe that's exactly what happened. According to Leland's rant Gallo had been playing games with the colonists for centuries.
  - Actually, he wasn't playing them for centuries, cause he just stayed for a few years and then went back to cryo sleep. The albinos in the film are the descendants of the colonists he twisted and sent to the cargo hold. The accelerants, meant to help humans evolve to Tanis' conditions, instead evolved the crazy colonists to the ship, and their descendants just followed after it.

The Tanis Colonists will evolve into fish-people

Well, they did land in an ocean. Land wasn't too far away but still they're going to be doing a lot of swimming salvaging the Elysium.

The whole film is the dream of Bower the night before the start of the mission.

His girl friend did just dump him this brought on a nightmare about how his mission could go. Think about it the film seems to be completely based around how sucky life could get for Bower and then he meets a beautiful girl and he saves her numerous times then they find out life is not all that bad and they get to live on a foreign planet for the rest of their life.

Alternatively, the whole film is a hallucination of Bower's brought on by Pandorum

Just like the last case, he began to believe the ship was 'cursed'. In his case, the curse was one of cannibalistic zombie goblins.

Earth was never destroyed

Gallo thought that it was as a result of Pandorum, and subsequently told everyone he released. He may even have killed his fellow crewmates specifically because he genuinelly thought they were crazy for denying that Earth was gone. There is, after all, little that could cause Earth to literally vanish from a map, and Gallo seems to be the only witness to that. Even if some horrible catastrophe did befall Earth, it could still be pandorum that let Gallo believe Earth was completely gone, which made him discount the possibility that the human race might have survived whatever it was and thus cemented his fall into nihilistic madness.
- What about the message? Leland had a record of it.
  - There must have been a serious disaster all right, and the old civilization may be gone, but it's hard to imagine how the entire planet could have ceased to exist.
  - Or the message was a deliberate hoax sent from Earth, because there'd been some kind of political upheaval back home, and the ones who'd seized power didn't want anyone from Tanis to ever try to return and overthrow their new empire. A similar message was disseminated to Earth's population at the same time, to say that the Elysium had been destroyed so no one living under the new regime would ever try to contact the colonists.